Method and device for producing fiber-reinforced components using an injection method

ABSTRACT

In a method for producing fiber-reinforced plastic components made of dry fiber composite preforms by an injection method for injecting matrix material, the arrangement of the fiber composite preform on one surface of the preform resulting in a flow promoting device, on a tool, creates a first space by a gas-permeable and matrix-material-impermeable membrane surrounding the preforms. Formation of a second space arranged between the first space and the surroundings by a foil, which is impermeable to gaseous material and matrix material, is provided, with removal by suction, of air from the second space resulting in matrix material being sucked from a reservoir into the evacuated first space and with the flow promoting device causing distribution of the matrix material above the surface of the preform facing the flow promoting device, thus causing the matrix material to penetrate the preform vertically.

FIELD OF THE INVENTION

The present invention relates to a method for producing fiber-reinforcedplastic components made of dry fiber composite preforms by an injectionmethod and subsequent low-pressure curing and to a device forimplementing this method.

BACKGROUND INFORMATION

Such methods use dry fiber composite preforms in order to producecomponents with geometric shapes that may be unwindable, non-unwindableor not completely unwindable. The dry fiber composite preform can be awoven fabric, a multi-axis interlaid scrim or a warp-thread reinforcedunidirectional preform. The above-mentioned preforms are used in theproduction of components made of fiber-reinforced material. Theyrepresent an intermediate process step before infiltration by resin andcuring occur.

Such a method is referred to as a so-called resin film infusion (RFI)method wherein dry carbon fibers, carbon fiber woven fabrics or carbonfiber interlaid scrim are placed in a curing device before a specifiednon-liquid quantity of resin film is applied to them from the outside.The curing tools equipped and evacuated in this way are subsequentlycured in an autoclave or another pressure receptacle by exposure totemperature and pressure. The use of pressure receptacles and theassociated complex tools that are necessary are however very expensive,rendering such methods complex also in regard to temperatures andpressures to be maintained. The scope of application of such methods isthus limited.

Furthermore, the use of dry preform components is described, forexample, in German Published Patent Application No. 198 13 105, whichdescribes a method for producing fiber composite components wherein thefibers and the matrix material are formed in a tool, forming a moldcavity, the tool including at least two parts, with the air situated inthe mold cavity being able to escape. In this arrangement, a porousmembrane is placed into the mold cavity, in front of the apertures, withthe pores of said porous membranes being of such a size that air can beevacuated without hindrance while the matrix material is retained in themold cavity.

The foregoing solution does not involve any application of pressure.However, it is associated with a disadvantage in that the size ofcomponents that can be produced with this method is limited, because thematrix material can be introduced into the fibers, i.e., into thepreforms, only in a limited way, provided a central matrix feed bush hasbeen provided, because the matrix has to flow along the preform plane,i.e., along the fibers. Due to the distance to be covered and theresistance put up by the material, this direction of flow creates thelargest flow resistance to the matrix. Thus, impregnation along thelength of material flow is limited. As an alternative, the matrix is putin place over an area. To this effect, resin reservoirs, situated on thecomponent surface, are used, which require their own expensive resinsupply device up to the preform, thus at every position posing the riskof a leakage (risk of rejects).

There is a further disadvantage in that this method can meet veryexacting quality standards of the component to be produced only to alimited extent. This is because as a result of the potential resinpassages through the vacuum foil and the membrane up to the preformsurface, matrix material can penetrate through the membrane in manylocations of the component, thus sealing off said membrane from above.In this case, air evacuation no longer functions and pores form withinthe laminate, because of the reaction during the curing process (e.g. asa result of trapped air, chemical separation, volatile components etc.).Such pores, which can negatively affect the quality of the component,cannot be eliminated.

Other conventional low-pressure methods, such as, for example, VARI(DLR) do without a membrane and two-part vacuum chambers. They avoidpore formation by process management of the vacuum and temperatureoutside the boiling range of the matrix material. In this way, no poresarise in the component. However, there is a disadvantage in thattemperature and vacuum management must be very exactly adhered to atevery position of the component, to avoid locally entering the boilingrange of the matrix, with subsequent local pore formation. In the caseof large components, such precise process management can only berealized with considerable difficulty and expense. This method has afurther disadvantage in that as a result of permanent suction tomaintain a vacuum, matrix material can be drawn from the component,which again can create pores. Furthermore, a resin trap or similar isnecessary so as to prevent damage to the vacuum pump as a result of anymatrix material issuing.

It is therefore an object of the present invention to provide a methodfor producing fiber-reinforced plastic components made of dry fibercomposite preform by an injection method and a device for implementingthe method, the method being suitable even for larger components, andallowing process management which is as simple as possible while at thesame time making it possible to achieve good component quality.

SUMMARY

The above and other beneficial objects of the present invention areachieved by providing a method and a device as described herein.

With the solution according to the present invention, it is possible toachieve top quality components. This is in particular advantageous inthe case of highly stressed structural carbon fiber reinforced plasticcomponents in the aircraft industry. Typical parameters indicating thequality of the components include, e.g., the number of pores within thecured carbon fiber reinforced plastic laminate and the temperatureresistance expressed in the glass-transition temperature of the matrixmaterial after the process.

The solution according to the present invention applies to theproduction of composite reinforced plastic components containing carbonfibers, glass fibers, aramide fibers, boron fibers or hybrid materials,the geometric shapes of which may be unwindable, non-unwindable or notcompletely unwindable. The solution is also suitable for the productionof non-stiffened or stiffened, large-area planking fields, plasticstools or tapered overlap repairs of damaged fiber composite components.Stiffening may be achieved by so-called integral stiffening (profilesmade of carbon fiber reinforced plastic, etc., profiles comprising acombination of sandwich and carbon fiber reinforced plastic, etc.) orstiffening may be achieved by a typical sheet-like sandwich structure.

The solution according to the present invention provides acost-effective method for producing fiber reinforced components,plastics tools and repair patches for tapered overlap repairs usingvacuum injection technology and curing in a vacuum, without the use ofan autoclave or without the use of overpressure.

The present invention is described below with reference to the severalFigures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view through a device according tothe present invention, the device being suitable to implement the methodaccording to the present invention.

FIG. 2 is a schematic cross-sectional view through another exampleembodiment of a device according to the present invention having atypical design of an integrally stiffened component as a sandwichhat-profile.

FIG. 3 is a schematic cross-sectional view through a further exampleembodiment of a device according to the present invention having atypical design of an integrally stiffened component as a T-profile.

FIG. 4 is a graph illustrating a typical temperature and vacuum gradientover time for a so-called 350° F. system.

FIG. 5 is a graph illustrating a typical temperature and vacuum gradientover time for a so-called room temperature (RT) system.

FIG. 6 is a schematic cross-sectional view of yet another exampleembodiment of the device according to the present invention.

DETAILED DESCRIPTION

In the device illustrated in FIG. 1, the component or dry fibercomposite preform 1 to be produced is arranged on a tool 3, for example,by a mounting 5. The component or laminate may be a reinforced plasticcomponent including carbon fibers, glass fibers, aramide fibers, boronfibers or hybrid materials, the geometric shape of which may beunwindable, non-unwindable or not completely unwindable. The componentor laminate is in particular suitable for the production ofnon-stiffened or stiffened, large-area planking fields, plastics toolsor tapered overlap repairs of damaged fiber composite components.Stiffening may be achieved by so-called integral stiffening (profilesmade of carbon fiber reinforced plastics, etc., profiles including acombination of sandwich and carbon fiber reinforced plastics, etc.) orstiffening may be achieved by a typical sheet-like sandwich structure.The shape of tool 2 is suitable for accommodating the component 1 or, ifnecessary, the mounting 5. The tool 2 may be made from various suitablematerials, e.g., wood, steel, sheet metal, glass, etc.

Component 1 is covered by a semi-permeable membrane 7, which isgas-permeable but which prevents penetration of matrix material. Outsidethe circumferential area 8, the membrane 7 is sealed as closely aspossible to the component 1 by a seal 9, which seals the first space 10formed by the membrane 7 and the mounting 5 or the tool surface 3. As analternative, the membrane 7 may also surround the entire component asillustrated in FIG. 6. This may be achieved by the seal 9 (FIG. 6) orwithout such a seal, by designing the membrane 7 in a single piece.Between the component 1 and the membrane 7, above the entire surface 11of the component 1 facing the membrane 7, a peel ply 13 (optional) and aspacer as a flow promoting device 15 may be arranged. The peel ply 13and the spacer serve to hold the membrane 7 at a distance from thesurface 11 of the component 1. The flow promoting device 15 may be atype of grate or screen or a stiff woven or knitted or braided fabricthat does not significantly compress when a vacuum is applied. Thefabric includes, for example, metal, plastics or semi-finished textilematerials.

The arrangement 17 including mounting 5, component 1, membrane 7 withseal 9 as well as peel ply 13 and flow promoting device 15, is coveredby a foil 19, which is impermeable to gas. Around the circumference ofthe membrane 7, the foil 19 is sealed on the tool 3 by a seal 21 so thatthe second space or interior space 27, which is formed by the surface 23of the tool 3 and the internal wall 26 of the foil 19, is sealed offfrom the surroundings. A ventilation fabric 32, for example, a wovenglass fabric, a fibrous web, etc., is placed between the foil 19 and themembrane 7. This ventilation fabric 32 leads the air and gasses, whichwere removed by suction through the membrane, from the interior space27, along the membrane surface, for removal by suction through thevacuum pump 29. This interior space 27 may be evacuated by a vacuum pump29 and a respective gas pipe 31 which leads into the interior space 27.In addition, a second pipe 33 leads into the interior space 27, throughwhich pipe 33 matrix material and in particular resin, may be introducedinto the interior space 27.

To feed matrix material into the component 1, hoses or pipes 33, whichare connected to a resin reservoir, lead into a space 25 arranged in thefirst space 10. The tool and the reservoir for the matrix material arelocated on hot plates, within a heated chamber, within a heatable liquid(oil bath, etc.) or within a controllable oven, if the selected resinsystem requires thermal treatment during injection.

The foil 19, the peel ply 13, the membrane 7, the ventilation fabric 32and the flow promoting device 15 all must be resistant, for the durationof the process, to the matrix systems used. In addition they must alsobe resistant to the temperatures that occur during the process.Depending on the particular geometric shape to be produced, placementonto such a shape by stretching, fold formation, etc. must be possible.

The foil 19 is a gas-impermeable state-of-the-art vacuum membrane withthe characteristics mentioned above. Its task is to seal off the secondspace 27 from the surroundings. Typical materials for this are foils orrubber membranes. Examples for a 180° C. (350° F.) application include,for example, foils based on PTFE, FEP, etc. other materials may beconsidered, depending on the selected matrix system and its specificcuring temperature, taking into account the above-mentionedrequirements.

The peel ply 13 serves to facilitate separation (by peeling), aftercompletion of the process, of the flow promoting device 15 filled withmatrix material from the component 1, because all the process materialsmentioned are only used as auxiliaries in the production of thecomponent 1. The peel ply 13 is configured to resist permanentconnection with the matrix material and the surface of the component.This is achieved by a particular surface structure of the peel plyand/or by additional non-stick coatings (such as, for example, PTFE,silicon, etc.). Typical materials are, for example, woven glass fabrics,woven nylon fabrics, etc. The peel ply must be gas-permeable and alsopermeable to matrix material in both direction.

The membrane 7 is a semi-permeable membrane, e.g., made of a technicalplastic material, which meets the process conditions as far astemperature resistance and media resistance are concerned. Furthermore,this membrane is gas-permeable but impermeable to liquids withviscosities that are comparable to water. This behavior is achieved bygas-permeable pores arranged in the membrane, the pores beingdistributed on the surface of the membrane over a greater or a lesserarea. The size of the pores is selected so that the matrix system cannotpenetrate them. The thickness of the membrane is in the range of tenthsof a millimeter. Adequate flexibility for draping and forming isprovided by the use of typical plastic materials.

The ventilation fabric 32 above the flow promoting device 15 serves toconvey the air and other volatile components sucked through themembrane, for removal by suction to the vacuum pump 29. This materialmay include any material as long as it provides adequate temperatureresistance and media resistance to the materials necessary during theprocess, and as long as the conveyance of air in longitudinal directionis possible. Fluffy mats, woven fabrics, knitted fabrics, braidedfabrics, etc. are used for this purpose, whereby the articles may bemade from metal, plastic or other materials.

The flow promoting device 15 enables distribution on the surface of thecomponent 1 of the matrix material which reached space 25 via the matrixsupply pipe. The flow promoting device 15 thus assumes the function of aflow channel. The flow promoting device 15 must maintain a minimumthickness when subjected to the vacuum build-up of foil 19 so as toenable such material flow. It is thus a spacer that forms a flow channelbetween the membrane 7 and the component 1. The flow promoting devicemay be a braided fabric, a woven fabric, a knitted fabric, etc., with,if at all possible, a wide-meshed structure so as to create little flowresistance. Any materials may be used, e.g., metal, plastic, etc., aslong as the above-mentioned common minimum requirements (temperature andmedia resistance) are met. To support the transport of the matrix, thematrix supply pipe 33 may reach as far as required into the first space10. One branch or several supply pipes are possible. Within the firstspace 10, this matrix supply pipe may include apertures, for example,holes, transverse slots, longitudinal slots, etc. These assist resintransport in the flow promoting device.

FIGS. 2 and 3 illustrate the device according to the present inventionas illustrated in FIG. 1, except that FIGS. 2 and 3 illustrate adifferent component 1. The reference numbers for components of the samefunction are the same in these Figures. The device according to thepresent invention is suitable for components of almost any shape. FIG. 2schematically illustrates a planking field (component 1) which in onedirection is stiffened by hat profiles. These hat profiles include afoam core 35 or a core formed from any material, with a closed surfaceand with dry fiber composite preforms 34 placed thereon, the dry fibercomposite preforms being hat-shaped. The fiber composite preforms 34 aremade from materials that are identical or similar to those of component1. The foam core 35 and the preforms 34 form part of component 1.

The component 1 illustrated in FIG. 3 is also a planking field which inlongitudinal direction is stiffened by one or several T-profiles 36.Component 1 which is to be produced as illustrated in FIG. 3 thusincludes the individual components 1 and 34. The T-profiles 34 are madefrom materials that are identical or similar to those of component 1. Inaddition, this component variant requires a support 37 for fixing thedry T-profiles 36 which in their non-impregnated state are unstable.These supports 37 may be made from typically rigid or semi-flexible toolmaterials such as, e.g., metal, wood, rubber, plastic, etc. Since thereis direct contact with the matrix material, this material of thesupports 37 must keep its form in relation to the matrix material duringthe process.

FIGS. 4 and 5 illustrate typical gradients of various resin systemclasses as a vacuum gradient 91 and a temperature gradient 92, with thegradient illustrated in FIG. 4 relating to a 350° F. system and thegradient illustrated in FIG. 5 relating to an RT-system.

The temperature and vacuum gradients may be broken down into at leasttwo phases, the injection phase 101 and the curing phase 103. Atempering phase 102 may be provided after these phases. In the injectionphase 101 the temperature is lower than in the curing phase 103.

The temperature gradient and the vacuum control are such that the curedcomponent is of optimum quality with few to no pores and a suitablefiber volume fraction being achieved. The specifications for temperatureare determined by the materials requirements of the matrix material.Irrespective of the matrix material selected, during the entire processright through to curing, i.e., the condition in which the matrixmaterial has changed its aggregate state from liquid to irreversiblysolid, the vacuum may be kept at a constant level. Normal values andtolerances that must be observed include for example 1 to 10 mbar(absolute pressure, near the ideal vacuum). After curing 103, it is nolonger necessary to maintain a vacuum. The necessary temperaturegradients are characterized as follows: during the injection phase 101at full vacuum, a temperature is required that is determined by theviscosity curve of the matrix material. The temperature is selected suchthat the matrix material becomes liquid enough to reach the interiorspace 25 via the supply pipe 33 by vacuum suction. This is the minimumtemperature necessary for the process. At the same time, thistemperature must not be so high as to cause curing (loss of viscosity,solid state of the matrix). Therefore (depending on the matrix materialselected), the process temperature is set such that injection ispossible (slight viscosity) and that the remaining time to curing forthe injection, i.e., near-complete filling of the interior space 25 withmatrix material is adequate (technical term, e.g., gel time). Typically,the necessary viscosities during the injection phase range, e.g., from 1to 1000 mPas. Typical temperatures for a 350° F. (180° C.) system are,e.g., 70 to 120° C. for the injection phase 101, approximately 100 to180° C. for the curing phase 103, and values of approximately 160 to210° C. for the tempering phase 102.

For selected matrix materials, e.g., RT matrix materials, the followingvariant is possible: injection temperature 101 equals curing temperature103 equals tempering temperature 102.

The vacuum is established before the injection phase 101 (FIG. 4) orbefore it. In the method according to the present invention, a vacuumthat typically ranges from 1 to 10 mbar, is generated for injection, thevacuum extending to completion of the curing phase. The vacuum shouldnot be reduced.

The method according to the present invention is described below:

Dry materials (e.g. carbon fiber reinforced interlaid scrim, wovenfabric, etc.) are positioned as specified in the design, and thus alaminate structure is formed from the individual layers of preform. Thetool has been pre-treated to separate, i.e., by release agents orseparating foil and peel ply (altogether this constitutes the design 5on the underside of component 1). This prevents sticking of the matrixmaterial to the tool and allows removal of the component (stripping)from the tool surface. The dry material of the component 1 may includethe peel ply 13. In addition, a so-called flow promoting device 15 issimply placed above this construction. In the case of complexcomponents, local lateral attachment, e.g. with temperature-resistantadhesive tape, may be provided. The membrane 7, which is air-permeablebut not liquid-permeable, is placed onto this flow promoting device 15and sealed off by the seal 21. Then, the ventilation fabric 32 is placedon the membrane 7 and sealed off from the surroundings by the foil 19and the seal 21. During this procedure, the matrix supply pipe 33 andthe vacuum pipe 29 are put in place with commercially available bushingsand seals as illustrated in FIG. 1.

After placement of the above-mentioned materials and the foil or vacuumfilm 19, the first space 10 is evacuated using the vacuum pump. At thesame time, a reservoir containing matrix material is connected to thesystem to introduce matrix material into the first space 10. The vacuumresults in a drop in pressure so that the matrix material is sucked fromthe reservoir into the evacuated first space 25. After this, the matrixmaterial flows through the flow promoting device 15 and the supply pipe33 and is distributed on the surface of the component, more or lessunhindered, and almost irrespective of its viscosity characteristics.Any air present is disposed of through the membrane 7, as a result ofpermanent evacuation, by suction, of the interior space 27. There is noflow of matrix material within the laminate construction, which ischaracterized by considerable flow resistance. Instead, the infiltrationof matrix material occurs from the component surface vertically downwardinto the laminate. The maximum flow path at each position of thecomponent is thus directly related to the component thickness at thispoint. The flow resistance is thus minimal. Consequently, it is possibleto use resin systems, which, due to their viscosity, were hithertounsuitable for infiltration, and it is possible to create components oflarge dimensions.

Membrane 7 serves the purpose of preventing the occurrence of local aircushions. If, for example, the flow fronts which form, close up,creating a closed air cushion in component 1 of the interior space 25without binding to the vacuum outflow of the air, no resin may flow intothis air cushion. A defect (no impregnation) would be the result. Theair-permeable membrane 7 prevents this effect because at every positionin the component, air may always move vertically to the surface, throughthe membrane, into a resin free space which may be ventilated, of thevacuum build-up 27. From there, above the membrane 7, the air is removedby suction, via the vacuum connection 29 by the ventilation fabric 32.The membrane is resin-impermeable. There is thus no need for monitoringthe flow fronts because the process of impregnation is self-regulating.The degree of impregnation is directly related to the quantity of resinsupplied and thus available to the process, as well as being directlyrelated to the quantity of fiber supplied.

As soon as complete impregnation has occurred, curing is performed at asuitable temperature while the vacuum is maintained at the same level.In conventional processes, the bubbles arising as a result of thechemical process (matrix boiling, volatile components, etc.) would leadto pore formation in the finished component. This is prevented by themembrane 7, because permanent ventilation vertical to the surface of thecomponent occurs through the membrane.

On completion of curing, the component may be stripped. This means thatall process materials are removed from component 1, e.g., by peelingthem off manually, and the component may be separated from the tool 3.Depending on requirements, the now stripped hard component with preformsimpregnated with matrix, may be subjected to a pure thermalafter-treatment (tempering in step 102). Tempering may also occur priorto stripping, but this is not necessary. Tempering after stripping willreduce the time during which the tool is tied up.

The maximum size of components that may be produced with the methodaccording to the present invention is almost unlimited. A natural upperlimit is more likely to be dictated by considerations associated withhandling of the component (transport, etc.) rather than with the methoditself. There is no minimum size for these components. The maximumachievable thickness depends on the resin types used and the availableinjection time. This injection time is determined by economic ratherthan technical limits. Other undesirable side effects, such as, forexample, an exothermal reaction during curing, depend only on the resinsystem rather than on the method.

In summary, the present invention relates to a method for producingfiber-reinforced plastic components made of dry fiber composite preformsby an injection method for injecting matrix material. In this method,removal by suction, of air from the second space 27 occurs, resulting ina pressure drop from the first space 10 to the second space 27, withmatrix material being sucked from the reservoir into the evacuated firstspace 10. Because of the flow promoting device 15, the matrix materialenters the preform 1 vertically, in a distributed manner, above thesurface 11 of the preform 1 facing the membrane 7. By combining thefunctions of distributing the matrix material above the componentsurface through the flow promoting device, and the possibility ofarea-like ventilation above the component, as well as the flow promotingdevice, through the membrane foil, the desired quality is achieved withcuring in a vacuum, without the use of overpressure.

1. A method for producing a fiber-reinforced plastic component made ofdry fiber composite preforms by an injection method for injecting matrixmaterial, comprising the steps of: arranging the fiber composite preformon a tool; creating a first space by a gas-permeable andmatrix-material-impermeable membrane arranged at least on one sidearound the preform, matrix material being feedable into the first space;creating a second space adjacent to the first space, the second spacebeing delimited from surroundings by a foil that is impermeable togaseous material and matrix material, the foil being sealed off from thetool; and removing by suction air from the second space, matrix materialbeing sucked from a reservoir into the evacuated first space, beingdistributed above the surface of the preform, and penetrating thepreform vertically.
 2. A device for producing fiber-reinforced plasticcomponents made of dry fiber composite performs by an injection methodfor injecting matrix material, comprising: a tool configured to arrangethe fiber composite preform; a gas-permeable andmatrix-material-impermeable membrane arranged at least on one sidearound the preform and creating a first space into which matrix materialis feedable; a flow promoting device arranged on a surface of thepreform; and a second space, sealed off from the tool, adjacent to thefirst space, the second space delimited from surroundings by a foil thatis impermeable to gaseous material and matrix material; wherein thedevice is configured so that removal by suction of air from the secondspace results in matrix material being sucked from a reservoir into theevacuated first space, the flow promoting device being configured tocause distribution of the matrix material above the surface of thepreform facing the flow promoting device, thereby causing the matrixmaterial to penetrate the preform vertically.
 3. The method according toclaim 1, further comprising sealing the first space by coupling thegas-permeable and matrix-material-impermeable membrane to the tool in anarea surrounding the preform.
 4. The device according to claim 2,wherein the first space is sealed by a coupling of the gas-permeable andmatrix-material-impermeable membrane to the tool in an area surroundingthe preform.
 5. The method according to claim 1, further comprising:arranging a flow promoting device on one surface of the preform, theflow promoting device causing distribution of the matrix material abovethe surface of the preform facing the flow promoting device.